Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
One field of application in which size and image quality is particularly important is medical applications (e.g., endoscopes). For medical applications the chip must typically be small while providing a high quality image. In order to achieve these characteristics, for a given chip size, the photosensitive apertures should be as large as possible, while peripheral circuitry should be as limited as possible.
Medical sensors often need to pass high quality image information over long wires (e.g., 4 m) to a remote unit (e.g., computer and display). These wires are typically relatively thin having a high resistance and capacitance compared to thicker coaxial cables used for televisions. As such, these thin cables are poor conductors of high frequency signals. Since these medical cables are relatively low pass filters with limited transmission bandwidth, sharp signal transitions (e.g., sharp rising/falling edges) are not effectively passed, resulting in a degraded final image quality.